1. Field of the Invention
The present invention relates to a facsimile apparatus which incorporates a magnetic tape recording and playback apparatus (referred to in the following simply as a magnetic tape recording apparatus) for storing digital data.
2. Description of the Related Art
A type of facsimile apparatus is known in the prior art which includes a magnetic tape recording apparatus, used to store digital image data, e.g. data which have been received via a telephone system and are to be subsequently supplied to a printer or other device. The basic concepts of Using a magnetic tape recording apparatus in a facsimile apparatus will be described referring to FIGS. 1 and 2. In FIG. 1, 30 denotes a magnetic tape of the magnetic tape recording apparatus, 31 denotes a magnetic recording/playback head, 32 denotes a playback circuit which converts a playback signal produced from the recording/playback head 31 into digital data, and 33 denotes a buffer memory which functions as an input buffer for data that are to be recorded on the magnetic tape 30 and also for data that are read out from the tape during playback (e.g. data that are to be transferred to a printer). It is assumed that the buffer memory 33 address space is divided into a plurality of sections of identical size (referred to in the following as records) for storing respective blocks of data of identical size. Data blocks A, B, and C recorded on the magnetic tape 30 are to be sequentially stored in respective records in the buffer memory 33. The tape motion direction is indicated by the arrow, and FIG. 1 shows the condition during playback operation, immediately before the data block A begins to be read from the magnetic tape 30. FIG. 2 shows the condition immediately following completion of reading the data block C from the magnetic tape 30. The data blocks A, B and C recorded on the magnetic tape have now been stored in the buffer memory 33 in respective sequential records which are numbered 0, 1, 2.
However such a type of digital magnetic tape recording apparatus has certain basic disadvantages by comparison with other types of storage device which can be used in a facsimile apparatus, such as a magnetic disk apparatus or a semiconductor memory. These disadvantages include a tendency for errors to arise due to the effects of dust particles on the head 31, the effects of temperature changes, electrical noise interference, etc. As a result, a higher rate of data errors occurs with such a digital magnetic tape recording apparatus by comparison with the other types of data storage apparatus.
Furthermore, the playback signal that is obtained from such a digital magnetic tape recording apparatus exhibits relatively large fluctuations in amplitude and time-axis variation components. Hence, once a data error has entered the playback data, it is difficult to execute processing for recovery from the error.
There are two basic types of error which can arise in the playback data. Firstly, due to the effects of dust particles on the recording/playback head for example, data drop out may occur, whereby a number of successive bits are lost from the playback data. Secondly, a number of successive spurious bits may be inserted into the playback data, for example due to the effects of electrical noise during the recording or playback process. FIG. 3 is a diagram to illustrate the effects of the latter type of error. It is assumed that erroneous bits have been inserted into the data recorded on the tape, e.g. due to noise, at the position on the tape indicated by numeral 29, within the data block B. As a result, the erroneous data are read out from the tape and stored in the section of the buffer memory 33 that has been predetermined for the record B, with the error data indicated by the hatched-line portion. However due to that error data being inserted in the playback data stream, the length of the data block B will be extended beyond its predetermined length (i.e. predetermined number of data periods). Hence as illustrated, the data block B will overlap into the buffer memory section that has been predetermined for storing the next record C. Thus the effects of the single error 29 will be successively propogated throughout the data records that are subsequently stored in the buffer memory 33, i.e. each of these records will not begin with the correct datum from the recorded data.
Assuming that the data consist of successive bytes representing respective picture elements of an image or a text page, then when the data shown in FIG. 3 are read out from the buffer memory and supplied as a data stream to a printer, the data from each of the records 2, 3, . . . will be incorrectly positioned along that data stream in relation to the data from record 0, so that the resultant printed image or text will be significantly distorted. That is to say, an entire picture portion which is generated from the data read from the remaining records 2, 3, . . . will be incorrectly displayed in relation to the picture portion which is generated from the data read from the record 0.
In general, data management information is recorded on the magnetic tape 30 together with the data, to define the respective start and end positions of each of the data A, B, C, . . . , for use in correctly storing these data in predetermined sections of the buffer memory 33 after read out from the magnetic tape. However such management information assumes that the respective data blocks A, B, C begin and end at specific predetermined positions along the tape. Thus the effects of an error such as that shown in FIG. 3 make it impossible to correctly apply such data management information to the playback data.
It will be understood that similar problems will arise when data are occasionally lost from the playback signal due to drop out, so that for example the data block B in the playback data might be made shorter than the predetermined length. In such a case too, the effects of the error will propogate into subsequent data records stored in the buffer memory 33.
It can be seen that such problems arise from the fact that due to such an error, the start and end points of each of the data blocks that are recovered from the magnetic tape to be stored as respective records and which occur subsequent to the error location, are no longer correctly defined.